Thermionic valve amplifier



Nov. 25, 1947.

THERMIONIC VALVE AMPLIFIER Filed June 22, 1943 P. K. CHATTERJEA ETAL 2 Sheets-Sheet 1 l Innors pam-'uma Komma @ATTBUEA CHARLES 'lilou/5f: 5am# Aitor ey Nov. 25, 1947. P. K. cHATTl-:RJEA ET AL 2,431,307

THERMIONIC VALVE AMPLIFIER Filed June 22, 1945 2 Sheets-Sheet 2 n-h-n--ug- Ihnen to P5 PQAFULLA Kumara CHATTE@ 09A Per@ Nv*- 2.51? 194" n `TnrmMroNIc VALVE AMPLIFIER e Praiulla Kumar Chatterjea and Charles Thomas Scully, London, ,-England, assignors, by mcsne assignments, to International Standard Electric Corporation, New Delaware York, N. Y., a corporation o'f Application June 422, 1943, Serial No. 491,769

In Great Britain July 23, 1942 The present invention relates to low frequency amplifiers and more particularly to those yadapted for direct currents. It employs, for coupling between stages, thermally sensitive resistance elements known as Thermistors 'Ihermistors have been in use yfor some years and are characterised by a temperature coeicient of resistance which may be either positive or negative and which is moreover many times the corresponding co-eicient for a pure metal such as copper. This property renders thermistors particularly suitable for a variety of special applications in electric circuits'. l

Various different materials are available for the resistance element of athermistor, these various materials having different properties in other respects; as one example, a resistance material having a high negative temperature co-eiiicient of resistance comprises a mixture of manganese oxide and nickel oxide, with or without the addition of certain other metallic oxides, themixture being suitably heat treated.

Thermistors have been employed in two different forms: (a) known as a directly heated thermistor and comprising a resistance element of the thermally sensitive resistance material provided with suitable lead-out lconductors or terminals, and (b) known as an indirectly heated thermistor comprising the element (a) provided in addition with a heating coil electrically kinsulated from the element. A directly heated thermistor is primarily intended to be controlled by the current which flows through it and which varies the temperature and also the resistance accordingly. Such a thermistor will also be affected by the temperature of its surroundings and may therefore be used for thermostati-c control and like purposes with or without direct heating .by the current flowing through it. An indirectly heated thermistor is chieily designed `tobe heated by a controlling current which ows through the heating coil and which will usually, but not necessarily, be diierent from the current which flows through the resistance element, but this type of thermistor may also be subjected to either or both the types of control applicable to a directly heated thermistor. n

A More detailed information on the properties of thermistors will be found in an article by G. L. Pearson in the Bell Laboratories Record, December 1940,page 106. Y

The object of the invention is to provide a means'for coupling two stages of anelectrical ampliiler intended for amplifying direct current land/or low frequency currents, and overcomes the 9 Claims. (Cl. 179-171) disadvantages associated with direct connections between the stages, for which it is usually necessary to provide neutralising batteries or the like to deal with the steady voltage or current present at the anode of a valve. The means whereby thesedimculties are avoided includes the use of an indirectly heated thermistor for coupling the stages, and at the same time any necessity for separate high tension supplied or separate grid biassing sources for the Various valves of the amplifier is avoided.

Another advantage of the method of the invention is that it may be easily arranged so that an ampliied output signal may have its polarity either the same as, or opposite to, that of the lnput signal, and simple switching arrangements can be provided to change the output polarity at will.

According to the invention, there is provided a thermionic valve amplifier comprising a rst stage and a second stage, and an indirectly heated thermistor, the resistance element of which is con-.l

nected to the control grid of the second stage, the heating coil being connected in series with the anode circuit of the iirst stage.

IZlhe invention will be more clearly understood from the following detailed description of embodiments which refer to the accompanying drawings in which- Fig. 1' shows a schematic circuit diagram of a two-stage ampliiier according to the invention;

Fig. 2 shows an amplifier similar to that of Fig. 1, but with alternative grid biasing arrangements;

Fig. 3 shows a modification of Fig. 1;

Fig. 4 shows a schematic circuit diagram of another embodiment of the invention; and Figs. 5 and 6 show amplitude and phase change characteristic curves applying to the ,amplifier of Fig. 4.

In this specification, all resistances not specifically referred to as thermistors are to be underystood to be ordinary resistances whose valves are substantially independent of the currents which flow through them. l In Fig. 1 there is shown a two-stage direct current orlow frequency amplifier comprising two valves V1 and V2 which are coupled together by means of an indirectly heated thermistor T1. The heating coil T1 of thermistor T1 is connected in series with the anode circuit of the valve V1 and the resistance element R1 of thermistor T1 is connected in series with a resistance Rti across source), 'I'he junction point of R1 and Rt1 is connected to the control grid of valve V1. The anode circuit of valve Vz is supplied from the positive high tension terminal-i-HT through the resistance 'R1 and the output is taken directly from the anode.

The control grid of valve V1 is biassed through the grid leak resistance R1: from a battery GBI.

If a positive potential be applied tothe control grid of V1 through the input terminal the anode current will increase, and will raise the temperature ofthe thermistor T1. II this thermistor has a negative temperature coefficient yof resistance, the resistance R1 will decrease, thereby making more negative the voltage applied to the grid of valve V2. This decreases the anode current o! Vz and increases the anode voltage. so that the effect of applying a positive signal to the valve.

V1 is to obtain an amplified positive signal from the output terminal connected to valve V2. II the thermistor T1 has a positive temperature coefficient of resistance, the reverse effect will occur, and an ampliiled negative signal will be obtained at the output terminal. If it should be desired to use such a thermistor and at the same time to obtain a positive output signal, the resistances R1 and Rt1 should be interchanged. It will be evident also that a simple switch could be provided for interchanging R1 and Rt1 to enable the polarity of the output signal to be reversed at will.

It will be observed that the use of the thermistor T1 permits a common high tension source to be-used for both the valves and no diiliculties regarding the potentials of the cathodes are introduced. Furthermore, the two grid biassing potentials could be obtained if desired from the same source.

The amplifier shown in Fig. 1 will be useful in a number of applications where it is desired to amplify direct currents, or currents of very low frequency which are to be used to operate apparatus of various kinds. For example, an ordinary direct current measuring instrument could be connected in series with Rr, or instead of it, or R1 could be replaced by any network containing a measuring instrument. Alternatively, a network containing a loud speaker or a recorder or the like could be connected instead oi R1 or to the output terminals of the ampliiler.

In another possible alternative arrangement the valve V1 could be omitted and a network containing a direct current instrument or other apparatus to be operated could be connected in place of the resistance Rti. In this case the thermistor Ti controls the instrument directly without the intervention of a second valve, and the diillculties associated with the high anode voltage, and the corresponding anode current are avoided.

Fig. 2 shows an amplifier similar to that shown 'in Fig. 1, but provided with alternative biassing arrangements tor the valves. The cathodes of V1 and V1 are in this case biassed positively by potentiometers R4, R14 and R1, R1 connected across the high tension supply. The thermistor resistance element R1 is in this case connected across Rs in series with a resistance Re, the control grid of V1 being connected to the junction of Re and R1. 'I'he valve V1 is provided with a grid leak resistance Ria.

When a positive signal is applied to the input terminal, the thermistor T1 will be heated up. and assuming it to have a negative temperatureV coefllcient of resistance the resistance R1 will fall making the control grid of Vz more negative to the cathode as before, and a positive signal is obtained from the output. By interchangingRi and Ra or by. using. a thermistor with a positive temperature coeiilcient', the polarity of the output signal will be reversed, and an interchanging switch can be provided it desired as ,explained above.

The resistances R14 and R11 could be omitted if desired, but the sensitivity of the arrangement would then be aiTected by variation in the cathode `currents caused by the signals. It is preferable to include these resistances and to choose their values together with the values of R4 and R5 so that the current flowing through them is relatively large compared with the cathode currents so that the effect of the variations of the latter due to signal amplification may be negligible.

Fig. 3 shows avmoditlcation of that portion of Fig. 1 which lies between the lines XX and YY. In this case there are two thermistors T1 and T: arranged in cascade so that the resistance element Rz and T: is connected across the resistance Rti, in series with a resistance Rtz. By this arrangement the potential of the control grid of valve V1 is subjected to a. double change produced by both the thermistors, whose heating coils are both in series with the anode circuit of valve V1; and this process could clearly be repeated as often as desired with a corresponding increase in gain of the ampliiler.

By an alternative and slightly simpler arrangement, the resistance element R1 of the second thermistor T2 may be simply connected in parallel with R1 (Fig. 1), the heating coils of the second thermistor being connected in series or in parallel. The second resistor Rt: shown in Fig. 3 is then not necessary. In this arrangement, or in that shown in Fig. 3, the thermistors may. of course, have diilerent characteristics.

Another embodiment of the invention is shown in Fig. 4, which represents an amplifier intended for a relatively wide band of frequencies including zero frequency, and may provide at the same time phase change correction at the low frequency end of ,the range. The ampliiler is shown with two stages comprising respectively the valves V: and V4, but there may be any number of other stages arranged in any well known way. The two valves are shown coupled by a circuit of any type represented by the block A suitably designed for the frequencies to be amplied. The circuit A may, for example, consist of a simple resistancecapacity coupling, or a transformer, or a filter, or it may comprise any number of stages of amplification coupled in any way, and employing the same supplies as for the valves V: or V4, or dinerent supplies.

The control grid circuit of valve V4 is made up of two resistances R11 and Riz in series, with the resistance element Ra of an indirectly heated thermistor T: connected in series with a grid biassing source GB! across R11.

The heating coil r: of the thermistor is connected in series with the anode circuit oi' the valve Va which contains also the anode resistance Ra. The anode resistance for valve V4 is Rio, and resistance R4 is shown connecting the control grid of valve V: to the biassing source GBI as before. The output is taken from the anode of valve V4.

It will be assumed that the coupling circuit A becomes inemcient towards the lower end of the frequency range, and i'ails altogether to transmit any direct current changes. It will be evident that the operation of the thermistor circuit in lthe addition Q1 theresistance Riz in 1118.4. which provides a. suitable load for the circuit A, but does not effect the working of the thermistor.

When an alternating current signal is applied tothe input of the amplifier shown in Fig. 4, an amplified signal will be applied to the coupling circuit A from the anode circuit of valve Va, and

.this will be transmitted to the valve V4 in thev usual way, after having been amplified (or attenuated), and suifered some phase change, according to the nature of the circuit A. 'I'his alternating current signal will also be transmitted to some extent (depending on the frequency) by the thermistor T2 with a retardation of phase also depending on the frequency. Thus, the signal obtained from the valve V4 will have reached this valve by both routes. the relative proportions of the signal transmitted by the two routes depending on the frequency.

The action of the circuit will be more clearly understood from the curves of Figs. 5 and 6. In Fig. 5, curve a shows the relation between the amplitude of the signal transmitted to the valve V4 by the thermistor Ta and the frequency, for a constant input signal. Curve b shows the corresponding Acurve for a suitable conventional circuit A. Curve c shows the combined effect on the valve V4. In Fig. 6, curves a, b and c show the corresponding phase changes. Curve a in Fig. 5 indicates that for very low frequencies the thermistor follows the current changes fairly closely, but as the frequency rises it becomes increasingly ineilicient and finally ceases to transmit anything at all. Curve b indicates the usual property of most coupling circuits or amplifiers which have a progressive reduction in eiiiciency below some minimum frequency.

The transmission through the amplifier of Fig. 4 may be conveniently considered separately in three frequency ranges, i, 2 and 3, as indicated in Figs. 5 and 6. In range i, the signals are transmitted principally through the thermistor. and in range 3 through the coupling circuit. In the intermediate range 2 the signals are conveyed jointly by both means, the chief proportion being transferred progressively from the thermistor to the coupling circuit as the frequency rises. It will be understood that the regions i, 2 and 3 will not in fact be so sharply defined as Figs. 5 and 6 indicate, these figures being intended only to be diagrammatic.

By suitably designing the thermistor Ta, and the circuit A, the overlapping portions in the region 2 may be made to compensate, and an overall transmission curve similar to c, Fig. 5, may be obtained.

Similar considerations apply to the phase change curves of Fig. 6. In region 2 curve a indicates that the/thermistor exhibits aprogressively increasing lag of the phase, while curve b shows the leading phase which increases as the frequency decreases, and which is characteristic of many coupling arrangements. By suitable design. phase compensation can be obtained in region 2, a curve like c, Fig. 6, being obtained for the combined arrangement.

While it is possible to obtain simultaneous compensation of amplitude and phase in the manner described, the two effects can be treated independently. For example, the thermistor may be included for the purpose of phase correction alone at the lower end of the frequency range ot an amplifier, which may not be required to trans- 'mit actually to zero frequency.

As the thermistors will be aected to some 'extent by changes in the ambient temperature, it

lmay be necessary to provide means in any well lmown manner for eliminatingor reducing this eilect if it should be appreciable.

Ii', as will frequently be the case, the apparatus incorporating the invention has to deal with signal input voltages which may be positive or negative with reference to the zero condition corresponding to the absence of signals, the arrangements must be such that the temperature reached by the thermistor for zero input should correspond to a suitable 'operating point of the resistance-temperature characteristic. Such a point will usually, but not necessarily, b e the mid-point of the straightest and/or steepest portion of the characteristic. The heating power should preferably be as large as possible compatible with this requirement in order to reduce the eifects'of changes in the ambient temperature. j

What is claimed is:

1. A thermionic valve amplifier including a first stage and a second stage. an indirectly heated thermistor having a heater and a resistance element, means for connecting the resistance element thereof between the control grid and cathode of said second stage, said means being capable of passing direct current, a source of D. C. voltage included in said connecting means, and means for connecting the heating coil of said thermistor in series with the anode circuit of the first stage, whereby currents of substantially zero frequency may be amplified.

2. A thermionic valve ampliiier according to claim 1, also including an additional frequency discriminating couple circuit for connecting the plate yof the iirst stage to the control grid of the second stage.

3. A thermionic valve amplier according to claim 1, in which the thermistor resistance element is connected in series between the control grid and the negative terminal of the bias source, and another resistance is provided connected between the control grid of the second stage and the positive terminal of said source.

4. A thermionic valve amplier including a first stage and a second stage, an indirectly heated thermistor including a heater and a resistance element, means for connecting said resistance element thereof to the control grid of said second stage, said means being capable of passing direct current, and means for connecting the heating coil of said thermistor in series with the anode circuit of the iirst stage, saidamplifier including a source of grid bias potential for said second staga resistance connected between the control grid of the second stage and the positive terminal of said source, and a connection including a resistance, placing said thermistor resistance element in series between said control grid and the positive terminal of said source, also including a second indirectly heated thermistor having a heater and a resistance element, means for connecting said resistance element of said second thermistor in series between the negative t 7 terminal of said source and the resistance ele- `ment of said nrs't thermistor, and means ior connecting the heating coil of said second thermistorA also in series with the anode circuit of said nrst stage.

5. A'therrnionic ampliner of two stages, including a coupling thermistor of the indirectly heated type having a resistor and a heater, connections placing said heater in the anode circuit oi' said rst stage, a connection from one terminal of said thermistor resistor to the grid of said second stage, a grid resistance connected between said grid and cathode of said second stage and a bias battery connected between said cathode and the other terminal oi said thermistor resistor, whereby said thermistor resistor and said grid resistance constitute a bias potentiometer for said second stage.

6. A thermionic amplifier of two stages, including a coupling thermistor of the indirectly heated type having a resistor and a heater. a source of bias potential for the second stage of said ampliiler, connections placing said heater in the anode circuit of said nrst stage, a connection from one terminal of said thermistor resistor to the grid of said second stage, a connection from the other terminal of said resistor to the negative terminal of said bias source, and a grid resistor connected between the positive terminal of said bias source and the control grid of said second stage. l

'1. A two-stage amplifier including a, coupling thermistor having a heater and a resistor, a source of anode potential, a nrst potentiometer across said source, a connection from the cathode of the iirst stage to a tap upon said potentiometer, a second potentiometer across said source, a connection from the cathode of the second stage to a tap on said potentiometer, connections placingl said heater in series with the anode oi the ilrst stage. a iixed resistor, connections placing said nxed resistor and said thermistor resistor in series between said second cathode and the negative terminal oi said source, and a connection iran the junction point of said nxed resistor and said thermistor resistor to the control grid of the second stage.

8. An ampliiler according to claim 1, additionally including frequency discriminating coupling means connected between the anode of said nrst stage and the control grid o! said second stage, whereby the rising amplitude-frequency characteristic of one coupling oisets the falling characteristic of the other coupling, so as to obtain substantially uniform amplincation over a wide range of frequencies.

9. An ampliiler of two stages, including irequency-discriminating coupling means connected from the output of the nrst to the input oi the second stage, a thermistor provided with a heater and a resistor, said heater being coupled in the anode circuit of the iirst stage. a potentiometer from grid to cathode of the second stage, a bias battery, and connections placing said thermistor resistor and said bias battery in series between a tap on said potentiometer and said cathode.

. PRAFULLA KUMAR CHATI'ERJEA.

REFERENCES CITED The iollowing references are oi record in the ille ot this patent:

UNITED STATES PATENTS ygs Number Name Date 2,204,962 Hildebrandt June 18, 1940 2,271,208 Sauer Jan. 27, 1942 2,017,192 Woln' Oct. 15. 1935 

